The structure and working principle of relay and its application in motor control, for example

A, the structure and working principle of relay figure l - 2 is a relay schematic structure, it is mainly composed of electromagnetic coil, core, contact and a reset spring. , there are two different relays in the coil when the power is in the off state of contact, called normally open contacts ( As shown in figure 1 - 2 the touch 3, 4) , in the closed state of contact, called normally closed contacts ( As shown in figure 1 - 2 contact in 1, 2) 。 When coil electricity electromagnet produce magnetic force, attracting armature, the normally closed contact disconnect, normally open contact closure. Coil current disappears, reposition spring armature to return to the original position, the normally open contacts disconnect, normally closed contact closure. Figure l - 2 b is a relay coil, normally open contacts and normally closed contacts in the circuit diagram symbols. A relay there may be some for normally open contacts and normally closed contacts. In the relay circuit diagram, generally with the same text symbols (composed of letters, Numbers, Such as KA2) To indicate the same relay coils and contacts. Second, the contactor in the motor control application is shown in figure 1 - 3 it is to use ac contactor control of asynchronous motor main circuit, control circuit and related waveform figure. The structure and working principle of contactor and relay are basically the same, the difference only lies in the rated current of the relay contact is small, the contactor is used to control high current load, for example, it can control the rated current of dozens of Ann to thousands of asynchronous motor. Press the starting button SBl, it normally open contacts connected, current through the SBl normally open contacts and stop button SB2, for overload protection heat relay FR the normally closed contact, through the ac contactor coils of KM, contactor of armature is attracted, that three of the main circuit of normally open contact closure, M of three-phase asynchronous motor power supply is switched on, the motor starts running, the control circuit in the contactor KM auxiliary normally open contacts connected at the same time. Let go after the start button, SBl normally open contacts disconnect, electricity flows through the KM of auxiliary normally open contacts and SB2, FR 'flows through the normally closed contact of KM coil, motor continue running. KM for the normally open contact of this function is called 'lock' or 'stay', it makes the relay circuit is similar to the R - S trigger memory function. In the motor run time press the stop button SB2, it normally closed contact of the disconnect, make KM coil electricity, Lord of KM off contact, three-phase asynchronous motor power supply is cut off, the control circuit of the motor to stop running at the same time I KM auxiliary normally open contacts disconnect. When the stop button SB2 is released, the normally closed contact closure, KM still lose electric coil, motor continue to stop running state. Figure 1. 3 shows the signal waveform figure, figure marked high level 1 status ( Coil electricity, button is pressed) With low level 0 state ( Coil power button is released) 。 Figure 1. 3 in the control circuit in the relay system and the PLC controller ladder diagram is used in great quantities, it is called 'starting - Keep - Stop 'circuit, or' stop 'bao for short circuit.
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